I'm a geochemist. In the past ten years I've fixed mass spectrometers, blasted sapphires with a laser beam, explored for uranium in a nature reserve, and measured growth patterns in fish ears, and helped design the next generation of the world's most advanced ion probe. My main interest is in-situ mass spectrometry, but I have a soft spot in my heart for thermodynamics, drillers, and cosmochemistry.

Friday, March 07, 2008

Chris and Ron are mostly right. The ‘red’ line is actually 4 overlapping lines, representing 4 different published values for the REE composition of CI chondrites. The blue line is the REE plot for an ordinary chondrite.

Chondrites are the original condensates from the solar nebula- the hot cloud of gas from which the sun and planets formed. As this cloud of gas cooled, the refractory (hard-to-evaporate) elements condensed to form minerals, and the minerals stuck together. There are a number of different types of chondrites, based on texture, temperature, etc.

Geochemists like the REEs (lanthanides). The reason for this is that in natural systems, rare earth elements all have broadly similar chemistry. As a result, changes in relative REE concentrations are easier to interpret than changes in 14 randomly selected elements.

Because scientists are interested in the relative changes, they like to normalize to an initial condition. And that’s where chondrites come in. Chondrites are the solar system’s initial condition- they represent the leftover raw material from which the terrestrial planets were assembled. They trouble is, they aren’t all the same.

Ordinary chondrites are the most common type of meteorite. They consist mostly of silicates and metal, and for most refactory (low vapor pressure) elements they have a composition that is similar to the sun. The generally have various metamorphic textures that indicate variable amounts of post-formational reheating. For volatile elements, ordinary chondrites generally show various degrees of depletion- those elements evaporated as the meteorite headed up.

CI chondrites are a rare type of chondrite. They contain lots of organic matter and structural water, and have no history of reheating. As a result, they have solar composition for almost all non-gaseous elements.

The trouble is, CI chondrites are rare. Of the 36,000 meteorites that have been found and catalogued, we have 5 CI’s.

Because the REE (lanthanides) are all refactory, many earlier papers and studies normalize to ordinary chondrites- they work fine. But as better measurements of CI chondrites became available, and as their importance was realized, most folks started normalizing to CI compositions instead. And for the unwary, this can cause complications.

This is because CI chondrites contain a large amount of water, sulphur, and organic material. As a result, the absolute REE concentrations are somewhat diluted, compared to ordinary chondrites. That makes perfect sense, and is no big deal, AS LONG AS YOU SPECIFY WHICH CHONDRITE YOU USED for normalization.

Let’s see how the professionals did.

This link is a REE search of the figure, report a total of 4 different reported values for normalization. They are:Anders & Grevese 1989 (2)McDonough & Sun 1995Wakita et al. 1971Anders & Grevese 1989 x 1.36Anders and Ebihara 1982

The 1.36 multiplication factor is used to calculate a volatile-free CI equivalent. In otherwords, if the CI had been a normal chondrite, that is what the concentration would be. The rationale behind this is explained in this lab’s website, and a decent compilation of early chondrite results is given.

Rimas et al. do not specify which type of chondrite they use for normalization. Floss et al. normalize to CI chondrite values, but their figure captions don’t say which Ci values they use. Both methods sections are blocked, so I can’t check to see if they say there.

Finally, here’s a table of all of the above, normalized to Sun & McDonough 1989:

Disclaimer:

All opinions, measurements, figures, and facts on this page are the personal opinions of Charles W. Magee, Jr, and do not represent the views of any of his employers: past, present, present-but-about-to-be-past, or future. None of the content herein has been subject to peer review, and should be treated with caution or derision. Any passing mention of OSHA code violations, criminal activities, unethical or unscientific behavior, or the clandestine Australian nuclear weapons program are fictions created to make rhetorical points, and do not represent the reality of my, or anyone else's, workplace. Do not attempt any scientific protocols described herein at home, with the exception of the chocolate chip cookie recipe. Do not apply the products of that protocol to individuals with heart disease, diabetes, high blood pressure or cholesterol, egg, wheat, dairy, or chocolate allergies. Do not view this blog continuously for more than 45 minutes without stretching and taking other precautions to prevent computer-related chronic injury.
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